The invention relates generally to power generation systems and more particularly to systems configured for supplying power from constrained power or energy sources.
Small and medium size generators are increasingly being considered for use in distribution networks because of the changing regulatory and market environment and because of the potential for energy savings and service improvements which may result from such use. A strong interest also exists in distributed generator (DG) systems that provide backup power capability and grid independent operation.
Grid independent operation occurs when a system of generators operates independently of the bulk power system to deliver power to loads at nominal voltage and frequency within an acceptable tolerance band. Grid independent operation can be extremely challenging for DG systems supplying dynamic loads such as motors, especially when non-conventional generators such as fuel cells, wind turbines, micro-turbines, or batteries, for example, are coupled with power electronic converters having limited current handling capability and no contribution to system inertia. In a traditional power system, when load demands are suddenly increased, the power required to support the load is instantly provided by the system inertia. The output of the power source is then increased to satisfy the load. For this reason, grid independent systems with limited or no physical inertia to support changing load conditions are difficult to operate successfully.
A mini-grid is a power system where a diverse set of local generation assets and controls are integrated to satisfy local loads. A mini-grid can be operated in a grid connected or islanded mode.
The voltage strength of a node in an AC power system where a mini-grid is connected can be estimated using the Short Circuit Ratio (SCR). SCR is defined as Ssc divided by SMG wherein, Ssc is the three-phase short circuit power at the node of connection when the mini-grid is disconnected (expressed in kVA), and SMG is the sum of the apparent power rating of the generation and energy storage equipment in the mini-grid (expressed in kVA). A node of a power system is defined to be weak if the SCR is less than 3.
The frequency strength of a power system can be estimated using the following ratio: Hsys divided by PMG, wherein Hsys is the rotational stored energy of the power system (expressed in kW·s); PMG is the sum of the active power rating of the generation and energy storage equipment in the mini-grid (Expressed in kW). A power system is defined to be weak if this ratio is less than 5 s.
It is desirable to have the capability to operate dynamic loads with constrained generation resources in weak or isolated grid networks and more specifically in low inertia mini-grid networks.